Solid fluxing practices for aluminum melting.
The use of flux is not well understood by some foundrymen. Given the number of flux materials available today and their varied uses, along with their melting temperatures, flux practice in aluminum melting is as complex as slag practice in ferrous metals. What many consider a black art is actually guided by scientific principles.
When adding solid flux materials to molten aluminum, every foundryman should understand the basic concepts of applications, handling and delivery, compositions, economics, as well as concerns about safety, disposal and casting quality. One method of delivering flux to the furnace is shown in Fig. 1.
Reasons for Fluxing
Fluxes should be used when melting aluminum because this alloy rapidly forms a layer of oxide (primarily alumina) on all surfaces exposed to an oxygen-containing atmosphere. Magnesium, a common alloying element in aluminum that also oxidizes rapidly, forms magnesium oxide.
Oxidation accelerates as temperature increases. Fine oxide particles in molten aluminum tend to remain suspended because its density is close to that of aluminum and its high specific surface area slows both flotation and settling. Moreover, oxides that separate from the melt tend to envelop substantial amounts of usable metallic aluminum.
Although fluxes offer several special purpose applications, the propensity for aluminum to oxidize is the main reason fluxes are used. Fluxing agents retard oxidation, accelerate inclusion removal, recover metallic aluminum from dross and clean oxide buildup from furnaces.
Melt cleanliness is increased by the proper use of flux. Although oxides are more dense than liquid aluminum, many accumulate at or near the melt surface due to surface tension effects and adsorbed (clinging) gases. Fluxes accelerate the inclusion separation process because they wet the oxides that they touch, increasing the effect of aluminum's surface tension that acts to repulse oxide inclusions from the melt.
Explained below are the fluxing applications foundrymen use for melt protection, separation of metals and oxides, furnace maintenance and melt refining.
Cover or Smelter's Fluxes--This type of fluxing is used during many melting operations, particularly those that involve highly oxidizing conditions, such as temperatures above 1430F (775C), melting chips and fines, or melting alloys containing Mg greater than 2 wt.%. The oxide film that covers the melting charge materials is in intimate contact with the melt. Cover fluxes strip the oxide shell from melting solids and also prevent oxidation of liquid aluminum by hot furnace gases. A thick flux layer also may be necessary to prevent burning of aluminum fines. Although the economic value of cover fluxes depends on each individual case, their use on rapid-oxidizing alloys is usually cost effective, especially with those containing more than 2 wt.% Mg.
Drossing Fluxes--Drossing is perhaps the most economically important flux application when recovering the metallic aluminum contained in dross. Dross may contain more than 80% metal suspended in less than 20% oxide. Melting compounds belong to this group, only differing by being added with the solid charge. Dross treated with these fluxes changes its appearance from wet to dry as its metallic aluminum content decreases.
To accelerate this process, drossing fluxes contain exothermic compounds that release oxygen and generate heat by combusting a portion of the metallic aluminum (Al and alloying elements such as Mg) in the dross. Drossing fluxes are added either by weight, about 0.2--1.0% of metal charged, or by melt surface area, 0.5 lb/|ft.sup.2~ (2.5 kg/|m.sup.2~. flux needed depends on cleanliness and contamination of the charge materials and how much dross is already present. The selection of flux and quantity is critical--too little exothermic combustion reduces fluxing efficiency. Too much flux (and flux that is too exothermic) burns excessively, creating excessive fumes and loss of usable metallic aluminum. The economics of metallic recovery will be discussed in detail later.
Cleaning Fluxes--This application keeps furnace or crucible walls above and below the melt line free of buildup, a major source of oxide particles that cause so-called hard spots in castings. Extreme cases of buildup can reduce furnace capacity. Although the oxide particles in buildup tend to be large enough to settle rapidly, they are frequently flushed into the casting if they enter the melt shortly before pouring.
Buildup begins as a composite of metallic aluminum and oxide so it may be loosened and dispersed with exothermic fluxes. It often originates from wet dross sticking to the furnace walls. Since buildup gradually increases its oxide content, forming corundum (a hard phase of |Al.sub.2~|O.sub.3~), it will eventually contain insufficient metallic aluminum to react with the flux. This is when a jackhammer is needed to remove the buildup.
Degassing Fluxes--These fluxes remove hydrogen and inclusions from the melt. Although flux degassing currently is not as efficient as sparging with gases (Ar, |N.sub.2~ and |Cl.sub.2~) dispersed with rotary impellers, it is often satisfactory and can be used to lower hydrogen contents prior to a final sparging gas treatment. Fluxes offer the advantage over inert gases of also removing inclusions. Degassing flux is most effective when injected in powder from beneath the melt surface, a new technology that is discussed later.
Fluxes can economically remove certain trace elements from the melt. Moreover, deliberate melt additions for metallurgical control also can be made. In fact, modification of an Al-Si alloy was originally discovered when Na was inadvertently introduced by a NaF-bearing flux. In general, melt additions are achieved with compounds that release Sr, Na, Ti, B or other elements into the melt. Although little or no fading occurs as long as the flux remains on the melt, the amount of addition may vary. Melt purification primarily consists of lowering the contents of alkali metals. These reactions, which often are unintentional, also are discussed later.
To ensure that flux does its job effectively, it must be applied to the melt in a TABULAR DATA OMITTED safe and efficient manner. The most common method is to manually shovel the flux, throwing it onto the melt surface or furnace walls. The amount of flux should be premeasured to reduce reliance on operator training and to improve material usage.
Equipment is available for blowing flux onto furnace walls. Melting compounds may be charged with returns or chips, either intermittently or as a continuous stream. Some cast houses introduce a flux into the stream of molten metal during tapping. This increases contact with the melt and can be programmed to give predetermined doses.
For flux to be most efficient, it should achieve the maximum contact with the metal, particularly for drossing. For best results the flux should be rabbled (stirred) into the melt, which brings the flux into contact with oxides that build up near the melt surface. (Cover fluxes should not be disturbed because they could create flux inclusions.) Afterwards, the metal should be left standing for 5-10 min to allow all of the flux to rise and the freed metallic aluminum to return to the melt. Finally, the inclusion-laden flux should be thoroughly skimmed off.
Exothermic wall cleaning flux is typically applied when the walls are as hot as possible to aid heating and softening of oxide buildup. A weekly practice consists of draining the furnace to its low level, coating the walls with enough flux to initiate a good reaction (a layer 0.12-0.25 in. thick), turning the burners on high for 10-15 min with the doors closed, thoroughly scraping off buildup and reacted oxide, and skimming debris from the melt surface. Holding furnaces should be held quiescently after skimming to settle any oxide particles that enter the melt.
A method to clean walls at normal temperatures is to add cleaning flux to the melt surface near the walls after skimming the melt, but before tapping the furnace. During tapping, the cleaning flux coats the wall as the melt level goes down. Any buildup on the walls reacts with the flux while the furnace is recharged and is scraped off easily during the next skimming operation. This method can be particularly convenient for preventive maintenance in melting furnaces, counteracting the sticking of wet dross to the walls.
New Delivery Method
In recent years, a new flux delivery system, known as flux injection, has been introduced. This method addresses the major drawback of conventional practices--limited contact of melt surfaces with unwanted impurities in the melt. Flux injection overcomes this limitation by delivering predetermined amounts of powdered flux beneath the melt surface. Upon leaving the lance, the flux melts into small droplets that offer a large specific surface area with the melt as they float to the surface. This accelerates flux-induced metal cleaning. Degassing occurs when injected fluxes release halide gases that sparge hydrogen.
Typical flux injection equipment includes a dry powder feeder that mixes powdered flux into an inert gas stream, carrying it through a lance immersed into the melt. Future developments will include hybrid equipment that combines the best traits of flux injection and spinning nozzle degassing.
Fluxes predominately are blends of chloride and fluoride salts with additives to instill special properties. Dyes often are added to differentiate products by color. Many combinations of ingredients are available, which impart different properties of fluidity, wettability and reactivity.
An important factor to consider is the flux's melting or reaction temperature range. A cover flux should be liquid at melt temperatures. Drossing and other exothermic fluxes should ignite at melt temperatures.
Most fluxes are a mixture of KCl and NaCl. These salts form a eutectic, which at 44% and 56%, respectively (9 wt.%), melts at about 1225F (665C). Another common ingredient in fluxes is NaF, which forms a ternary eutectic trough with KCl and NaCl that dips to 1125F (607C). A common cover flux contains about 47.5 wt.% KCl, 47.5 wt.% NaCl and 5 wt.% NaF.
Other cover fluxes are based on Mg|Cl.sub.2~ and KCl, which form a low melting eutectic--800F (425C), or Mg|Cl.sub.2~xKCl (carnallite), which melts at a slightly higher temperature--910F (485C). These cover fluxes have high fluidity and can form thin layers on the melt surface. In addition, Mg|Cl.sub.2~ is used as a fluidizing addition. However, Mg|Cl.sub.2~ is an expensive ingredient, so it is used primarily in Na-free fluxes for alloys containing more than 2 wt.% Mg.
Alkali fluoride salts act as surfactants, decreasing the surface tension between the molten flux and molten aluminum, as well as between the molten flux and oxides. Chloride salts exhibit this property to a lesser degree. Wettability favors separation of oxide inclusions from the melt and metallic aluminum from the dross. Moreover, alkali fluoride salts have limited solubility for oxides, which facilitates penetration into oxide films that contain metallic aluminum in dross and buildup. Most fluxes contain fluoride salts, such as cryolite (|Na.sub.3~|AlF.sub.6~), calcium fluoride (Ca|F.sub.2~), and sodium silicofluoride (|Na.sub.2~|SiF.sub.6~), in amounts up to 20%. Unfortunately, the high melting points of alkali fluoride salts make them thicken liquid flux, limiting their use.
The addition of several percent of oxygen-containing compounds such as KN|O.sub.3~ increases the exothermicity of fluxes. Released oxygen reacts with metallic aluminum yielding |Al.sub.2~|O.sub.3~ and considerable heat. This locally increases the temperature, producing extra fluidity in both molten aluminum and flux, which, in drossing fluxes, enhances recovery of metallics suspended in oxide. In cleaning fluxes, the reaction increases penetration of the flux into buildup.
Certain compounds decompose into chlorine, carbon dioxide or metal halide gases. If they are introduced beneath the melt surface, they create bubbles that remove hydrogen. The most notable gas-releasing compound is hexachloroethane (|C.sub.2~|Cl.sub.6~), which generates |Cl.sub.2~ and gaseous aluminum chloride (Al|Cl.sub.3~), but the list also includes certain carbonates. Also available are proprietary fluxes that release metal halide gases without forming noxious |Cl.sub.2~.
Compounds that react with aluminum or its impurities can be used to add certain elements to the melt or reduce the concentration of others. For example, NaF will add traces of Na to the melt. Likewise, |K.sub.2~Ti|F.sub.6~ can add Ti and KB|F.sub.4~ can add B. To some extent Al|F.sub.3~ removes Ca, Sr and Mg, and Cl-releasing compounds remove Ca, Li, Mg, Na and Sr. On the other hand, P additions are made with flux containing amorphous phosphorus.
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|Title Annotation:||part 1|
|Date:||Jul 1, 1992|
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